4 research outputs found
Particulate contribution to extinction of visible radiation: Pollution, haze, and fog
No full textInternational audienceA data set acquired by eight particle-dedicated instruments set up on the SIRTA (Site Instrumental de Recherche par Télédétection Atmosphérique, which is French for Instrumented Site for Atmospheric Remote Sensing Research) during the ParisFog field campaign are exploited to document microphysical properties of particles contributing to extinction of visible radiation in variable situations. The study focuses on a 48-hour period when atmospheric conditions are highly variable: relative humidity changes between 50 and 100%, visibility ranges between 65 and 35 000 m, the site is either downwind the Paris area either under maritime influence. A dense and homogeneous fog formed during the night by radiative cooling. In 6 h, visibility decreased down from 30 000 m in the clear-sky regime to 65 m within the fog, because of advected urban pollution (factor 3 to 4 in visibility reduction), aerosol hydration (factor 20) and aerosol activation (factor 6). Computations of aerosol optical properties, based on Mie theory, show that extinction in clear-sky regime is due equally to the ultrafine modes and to the accumulation mode. Extinction by haze is due to hydrated aerosol particles distributed in the accumulation mode, defined by a geometric mean diameter of 0.6 μm and a geometric standard deviation of 1.4. These hydrated aerosol particles still contribute by 20 ± 10% to extinction in the fog. The complementary extinction is due to fog droplets distributed around the geometric mean diameter of 3.2 μm with a geometric standard deviation of 1.5 during the first fog development stage. The study also shows that the experimental set-up could not count all fog droplets during the second and third fog development stages
Extinction of Light during the Fog Life Cycle: a Result from the ParisFog Experiment
No full textInternational audienceData set acquired by five particle-dedicated instruments set up on the SIRTA experimental site during the ParisFog field campaign are exploited to document microphysical properties of particles contributing to extinction of visible radiation in variable situations. The case study is a 48-hour period when atmospheric conditions are highly variable: relative humidity changes between 50 and 100%, visibility ranges between 35000 and 65 m, the site is either downwind Paris area either under maritime influence. A dense and homogeneous fog formed by radiative cooling during the 18-19 February night. In 7 hours, visibility decreases from 26 000 m to 65 m, because of transported pollution (factor 3 in visibility reduction), aerosol hydration (factor 20) and aerosol activation (factor 6). According to Mie theory, extinction in clear-sky polluted and unpolluted regimes is due equally to Aitken and accumulation modes. Extinction in haze is due to hydrated aerosols distributed in the accumulation mode, for diameter smaller than 2 ÎĽm. Hydrated aerosols of the accumulation mode still contribute to 20-30% extinction in the fog. Measurements show that fog droplets, with diameter included between 2 and 10 ÎĽm, contribute to 40% extinction during the first hours of the fog
